Input system for electrical amplifiers



Jan. 26, 1932.

Fileq June 16, 1928 3 Sheets-Sheet 1 f/er.

Zjmentb:

Jan. 26, 1932.

F. H. DRAKE ET AL. 1,843,018

INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Filed June 16, 1928 s Sheets- Sheet 3 XII/#20004 Patented Jan. 26, 1932 PATENT OFFICE FREDERICK H. DRAKE AND WILLIAM I). LOUGHLIN, OF BOONTON, NEW JERSEY, AS-

SIG-NORS, BY MESNE ASSIGNMENTS, TO RADIO CORPORATION OF AMERICA, 01

NEW YORK, N. Y., A CORPORATION OF DELAWARE INPUT SYSTEM FOR ELECTRICAL AMPLIFIERS Application filed June 16,

This invention relates to electrical amplifiner circuits, and more especially to an input system for such amplifier circuits. It is particularly applicable to aninput system for a radio frequency amplifier of a type suitable for use in radio receiving sets, and the invention will hereafter be described in connection with such an amplifier, in which'connection it has many advantages.

An object of the invention is to provide such aninput system constituting a desirable means for coupling an antenna or other collecting device to a radio frequency amplifier. A further object is to provide such an input system including a volume control. Other objects of the invention will appear from the following description.

Figure 1 is a diagram of one form of in- I put' system embodying the invention and showing the antenna and ground connections. Figure 2 shows a similar input system including a volume control. Figure 8 shows a multi-stage radio frequency amplifier including an input system similar to that of Figure 2. Figure 4. shows one construction of a trimmer for use in an input system according to the invention. Figure is a graph showing curves of the wavelength characteristic-of certain input systems.

Referring now to Figure 1, the input system comprlses a tuned circuit including in seriesa fixed condenser Om, an inductance L0 of adjustable value (1. e. a tr mmable inductance or tr1mmer) and a'va'riable tuning condenser C0 connected across said fixed condenser and trimmable inductance. The terminals 1 and 2 of the tuning condenser O0 may suitably constitute the input terminals of a radio frequency amplifier, fol-example the grid and cathode of the first tube. The antenna may be connected or coupled to a'point between L0 and Cm, the other side of Cm being grounded.

Although theinvention is not limited to any particular values for the several circuit 1928. Serial No. 285,886.

elements, the following approximate values (here given as an example only) have been successfully employed:

m 1000 ##f 0,, (max) =40O ,upzf

0 170240 m. h. (trimming range) In other cases a value of 0 =2000 a ,u. f has been employed with good results.

It will be observed that the total tuning capacity of the input system, with the antenna removed, is

The antenna capacity is effectively in parallel with C and acts to increase the effective value of C in the above equation.

If C is large compared with C as is the case when the circuit is tuned to short wavelengths, the term (i. e. is negligible in comparison with 1) for example at short wavelengths. At the tion and for atypical input system according to one of the most widely used prior art arsame time, at short Wavelengths the voltage, 6, between antenna and ground, is applied to the tuned circuit L C -G by means of the coupling between that circuit and the antenna through the capacity C C -being fixed, this coupling decreases at short wavelengths, and increases at long wavelengths. As a result of this arrangement, the wavelength characteristic of the input system is of more desirable form than has beenhitherto obtainable, and by proper proportioning of the constants of this circuit, the wavelength characteristic may be made uniform or substantiallyconstant over "the wave-" length range: for I which the system is designed.

Figure 5 shows certain wavelength characteristic curves, obtained by plotting ng/ (i. e. the ratio of the voltage 6 induced in the, antenna by the signal, to the voltage E across the'tuning condenser C against wavelength (A) in the broadcastrrange) fora typ cal input system according to the present invenrangements. Curve B is afcharacteristic wrv of against wavelengthwfor an input system. ac-

j cording to the present invention, as obtained indoor antenna approximately feet in {5,3

with antenna A which-was a single-room length and 100 f capacity, Curve B is a typical characteristic curve,'obtained with the same antenna A for an input system aocording to the prior art, in which the antenna is coupled'to a tuned input circuiteither through a small series capacity, or through a small coil coupled to a coil of the tuned input circuit, either prior art arrangement resulting in substantially the same characteristic curve. Curves C and C are similar curves for the same inputsystemsbut were obtained with antenna A which" was a short outdoor antenna of approximately 250 7 capacity.

- In the case of, input circuits ofthe type involving finductive. coupling and fixed inductances' tuned to resonance at" difierent wavelengths by" variable" condensers, as has heretoforebeen the usual arrangement, the

wavelength i'charact'eristic curve ordinarily has the shape of curvesBand C? of Figure 5. This shape of vcharacteristic curve is undesirable, especially in thecase of input systems for use with radio frequency amplifiers, for anumber oat-reasons. For exam le, such amplifiers are muchfmore -apt to be unstable .at short wavelengths than at" long wavelengths, due to the increased capacity and magnetic couplings inthe amplifier at short wavelengths. To avoid instability, excessive sensitivity, or tendency' to oscillate, it is frequently desirable, to impart to such an aniplifier, if possible, an amplification characteristic curve which is substantially constant over a range of wavelengths, or which at least has a minimized rise at short wavelengths. Whether or not such a characteristic is imparted to the amplifier, it is'highly desirable that the input voltage impressed upon the input terminals of the radio frequency amplifier should not be very greatly higher at short wavelengths than at long wavelengths, and it is preferable that-the voltage should be substantially constant over the wavelength range of the amplifier, since otherwise the unc'ler'sir'able sloping characteristic of the radio frequency amplifier, alreadyhigher at short wavelengths than at long wavelengths,

willhave superimposed upon it another undesirable slopingcharacteristic of the same type, making an overra-ll amplification which is" so'much higher atshort than at long wavelengths as to produce oscillation, instability, excessive sensitiv ty and other undesirable efiects, For the purpose of avoiding this condition, it is desirable therefore to employ an input system having a wavelength char .acteristic curve resembling the curves shown at B and of Figure 5,;which curves were obtained with an input system according to the present invention .designed'for use in a broadcast radio receiving set,and which are substantially constant over the broadcast range of. wavelengths from 200:550 meters. Such wavelengthcharacteristics are possessed by input systems according .to the present invention, and constitute an important adva n-. tage thereof. f e

Figure 2 shows the invention applied to an input system including a volume control arrangement as anintegral. part thereof. This is provided by the resistance R connected across the fixed capacity C and provided with a variable tap connected to the antenna.

When this antenna tap is at the top of the resistance R, the antenna has maximum coupling tothe input circuit through O, so that the full incoming signal is impressed upon succeeding amplifier, and as the tap is moved down'along R the antenna is connected more and more nearly directly toground, so that less andless of the received signal is impressedupon an amplifier with which the input system is associated, At the bottomposition of the tap, the antenna is grounded, and there substantially no coupling to the input system. 7 V v In one such arrangement, which has been found to give an excellent volume control effect, the resistance It had a value of150,000 ohms when the other constants ofthe'input systeinwere as givenfin the above example in connection withFigure 1. l i 5 {It is to; be noted 'that'the .volume control arrangement described above is not of general applicability to tuned input systems of the types hitherto employed, but canonly be is very small as compared to the value of the fixed' capac-ity C The adjustment of the resistance R therefor exercises substantially no detuning efiect.

Figure 3 shows the combination of an input system and volume control arrangement according to the present invention with a tuned radio frequency amplifier of the type described and claimed inUnited States Patent No. 1,672,811, issued June 5, 1928. In

this arrangement T T and T are three electron or vacuum tubes in three radio frequency amplifier stages working into a detector stage including tube T An input system L C -C R according to the present invention is arranged between grid G and cathode F (which may be grounded as shown) of the tube T in the first radio frequency stage. Each tube has a plate coil L which is coupled or connected so as to have effectively negative mutual inductancewith the secondary coil L In series with L is connected a tertiary coil L having negligible coupling with the coils L and L, for which purpose it may be arranged at right angles thereto, and atuning condenser C A balancing capacity is connected between the control grid G and the junction of coils L and L while the cathode F is connected (ordinarily through ground) to the junction of the coil L and the tuning condenser C Common A and B batteries, or equivalent socket-power de-' vices may be employed for the several stages, being suitably by-passed if desired or alternating current filaments or cathodes of the separate heater types may be employed; and the grids G may, if desired, be biased by means of a suitably arranged O battery or by a biasing resistance in, the filament circuit. The detector stage, including tube T may be arranged in any suitable way, being provided for instance, with grid condenser C grid-resistance H and radio frequency by-pass condenser C, the detector output terminals being suitably connected to an audio frequency amplifier and loud speaker.

Such an arrangement constitutes a balanced multi-stage amplifier, as described and claimed in above mentioned United States Patent No. 1,672,811, and is adapted for operation by a single control, all of the condensers C C C 0 being units of a gang. They are then operated together, and it is desirable that for any given motion of the single control which operates these condensers, each condenser shall have the same increment or decrement of capacity as all the other conden sers. When such is the case, all of the circuits L -L C tune together and resonate to the same frequency. In order to make the cir cuit L -C C R tune and resonate with the other tuned circuits, i. e. so that the circuits tuned by condensers C and C track or remaln in alignment, the inductance L may be trimmed or adjusted, when necessary,

by a suitable trimming adjustment, thereby obtaining exact resonance of the input system. This trimming adjustment may be employed either to take care of different antennas or other collecting devices employed in connection with the input system (as, for example, when a change is made from antenna A to antenna A of Figure 5, or when a single model of broadcast radio receiving set is sold to many different purchasers each erecting his own collecting device) or, in tuning to different wavelengths on a single antenna, to keep the input system in resonance at the same wavelength as the other tuned circuits. Thus the trimming of L, takes care of departures in the value of C from substantial equality with the value of C for values of C, where (l /(l is no longer negligible compared with unity. c

' In such an arrangement as that of Figure 3, when the radio frequency amplifier is embodied in a radio receiving set, the set normally possesses three panelcontrols; namely a main tuning control which operates the condensers G C C C as a gang, a trimmer which varies the inductance of L and a volume control which operates the tap on R.

Such a control system for a radio receiving set has been found to he unusually satisfactory, and is now widely used commercially.

The particular multi-stage arrangement of Figure 3 is here shown by way of example only of the combination of an input system with a single-control multi-stage tuned radio frequency amplifier, according to the present invention, and the invention is, of course, not limited thereto.

One suitable arrangement and construction for such a trimmer is shown in Figure 4, in which the coil L is wound on a cylindrical form and mounted on the chassis or panel of a radio frequency amplifier. The inductance of the coil L is adjusted or trimmed by means of the conducting sleeve 8, suitably composed of copper or the like. This sleeve is moved back and forth over the coil L so as to surround more or less of it, bymeans of the push rod 4 moving in guide 5, and operated by the lever or crank 6 and control knob 7. In one such construction which has been successfully employed, the coil L0 had a diameter of approximately one inch, being composed of about 135 turns of #34 enamel wire, and the movable shield? had a diameter of about 1 This was found to give a decrease in the inductance of coil L of about 30% of its maximum value when the shield 3 was moved so as to substantially cover the coil.

, In addition to the advantages already described as resulting cording to the present invention, the follow the amplifier it if from an input system ac- 

